To assess the impact of a culturally proficient dietary/physical activity intervention on changes in body mass index (BMI) (kg/m 2 ).Randomized controlled trial (Hip-Hop to Health Jr.) conducted between September 1999 and June 2002 in 12 Head Start preschool programs in Chicago, Illinois.Intervention children had significantly smaller increases in BMI compared with control children at 1-year follow-up, 0.06 vs 0.59 kg/m 2 ; difference −0.53 kg/m 2 (95% CI −0.91 to −0.14), P=.01; and at 2-year follow-up, 0.54 vs 1.08 kg/m 2 ; difference −0.54 kg/m 2 (95% CI −0.98 to −0.10), P=.02, with adjustment for baseline age and BMI. The only significant difference between intervention and control children in food intake/physical activity was the Year 1 difference in percent of calories from saturated fat, 11.6% vs 12.8% (P=.002).Hip-Hop to Health Jr. was effective in reducing subsequent increases in BMI in preschool children. This represents a promising approach to prevention of overweight among minority children in the preschool years.